As technology improves and more advanced testing is carried out, there is an increasing awareness that the ground around us may be permeated by gases such as radon or hydrocarbons which are injurious to health or may cause an explosion if allowed to concentrate in enclosed volumes, such as buildings. Such gases may be naturally occurring or produced for example by reactions in landfill sites.
Particularly in the latter case, there is a need to ensure, before the sites are commissioned, that measures are taken to prevent the gases from escaping into the surrounding terrain, both for safety reasons and because the gases are themselves a valuable asset, being usable as a fuel to generate power.
One measure which is currently used to prevent the spread and accumulation of underground gases is to dig a trench along a selected boundary of the contaminated ground, to line the side of the trench opposite the contaminated ground with gas-impermeable sheeting and to fill the trench with loose aggregate to provide a vent. The intention is that any gases reaching the trench will permeate up through the aggregate and will be dispersed into the atmosphere along its entire length, avoiding any build up in a particular area and preventing the further spread of the gases.
A problem which is found with this arrangement is that trenches tend to silt up over long periods of time to the extent that the aggregate loses its permeability in certain areas and the gases tend to be channelled along defined routes to exit from the trench at discrete locations rather than along its entire surface. Thus, in time, the trench promotes a concentration of gas in the atmosphere at certain locations, exactly the effect that it is intended to stop.
A further problem with the use of trenches is that, being designed to disperse gases into the atmosphere they do not assist in the collection of gases which are usable as a fuel or which, because of their harmful nature, should be contained rather than discharged.